1. Field of the Invention
The present invention relates to a method and apparatus which inspect a bent portion of a wire bonded to, for example, semiconductor devices.
2. Prior Art
In the manufacture of semiconductor devices such as integrated circuits, etc., there is a wire bonding step. By the use of a wire bonding machine, wires 3, as shown in FIG. 4, are connected between the pads la of a pellet 1 and the leads 2a of a lead frame 2.
One example of a wire bonding machine is shown in FIG. 3.
The bonding machine 7 includes a bonding head 11 which is mounted on an XY table 10. The XY table 10 is driven in the X and Y directions by an X-axis motor 8 and a Y-axis motor 9. A bonding arm 12 is installed on the bonding head 11 and moved up and down by a Z-axis driver (not shown) that is installed in the bonding head 11. A bonding tool 13 is mounted at the tip end of the bonding arm 12, and a bonding wire (not shown) passes through this bonding tool 13. A camera holder 14 is mounted on the XY table 10, and a camera 15 is attached to this camera holder 14 with an off-set distance L between the camera 15 and the bonding tool 13. Workpieces are monitored by the camera 15, and the images of the workpieces are displayed on a monitor 17.
Workpieces are intermittently fed by a feeder 16, and the discrepancy between the position of each workpiece and a predetermined standard position is measured at least two points on a wire by the camera 15, and bonded point coordinate data is corrected in accordance with the measurement results. The distance L is added to the thus obtained corrected bonded point coordinate data, and the bonding tool 13 is moved accordingly, so that the bonding is performed onto the wires 3 as shown in FIG. 4.
In recent years, the distance between adjacent wires has generally becomes smaller as a result of increasing demand of fine pitch in semiconductor devices. As a result, there has been an increased danger of wires causing short circuits. For this reason, tolerance values of the bent portions (called "bends") in the wire loops have become extremely strict, and it is essential to measure and know exactly how large the bend of a wire loop is so that an adjustment is made accordingly in the wire bonding machine.
Conventionally, the bent portions in wire loops are inspected in the following manner: A workpiece with wires bonded to it is removed from a bonding machine and taken to a measuring instrument. As shown in FIG. 5, the distance S of the point 3c, which is the outermost point of the bend of the wire 3 between the first bonded point 3a and the second bonded point 3b from a straight line 4 which connects the first and second bonded points 3a and 3b, is observed and measured by microscopes. Since the maximum degree of bending is generally at the midpoint between two points (the first and second bonded points 3a and 3b in this case), the midpoint of the loop is selected as the outermost point 3c.
However, when the workpiece is removed from the bonding machine and taken to the measuring instrument, it might happen that the lead frame 2 is bent. If this occurs, the distances between the pads 1a of the pellet 1 and the leads 2a of the lead frame 2 change. This results in that the bend of the loop is further bent because the bent portion is pressed or elongated when the workpiece is bent.
Thus, according to the prior art inspection method, the bend of a wire loop is measured with an addition of the length caused by the deformation of the loop, and accurate measurements cannot be obtained. Furthermore, since the measurements are performed by hand, it requires a skilled operator and a long measurement time.